Doped tungsten filaments have been used for many years in incandescent lamps since Aladar Pacz was issued a patent, U.S. Pat. No. 1,410,499 on Mar. 21, 1922 for his discovery that adding very small quantities of an alkali silicate to tungsten a filament could be produced that would not offset nor sag.
Doped tungsten is normally fabricated from doped tungsten powder by a powder metallurgical process. In the process high purity tungsten oxide prepared from tungsten ore is doped with small concentrations of potassium, aluminum, and silicon compounds. The doped tungsten oxide powder is then reduced to tungsten metal powder, which is subsequently compacted and sintered into a high-density ingot. During the sintering process most of the doping substances evaporate and only small traces of certain doping elements remain in the ingot. The ingot is then swaged down to rods, which are then drawn into fine wires. During the process the doping material is distributed in arrays of fine bubbles which are stabilized at high temperatures by vapors of the doping substance. The fine bubbles considerably affect the recrystallization behavior of the heavily deformed tungsten wire, resulting in the desired high-temperature properties. The problem with this conventional tungsten preparation process is that an excess amount of doping elements are introduced and must be removed during the sintering treatment to obtain good wire properties. The conventional process also tends to produce non-uniformities in dopant distribution resulting from non-uniformities in initial additions or non-uniform removal during sintering. This can lead to inhomogeneities in the wire properties which may give rise to localized sagging to hot-spot formation.
In U.S. Pat. No. 3,820,868, dated June 28, 1974 issued to Sell et al is disclosed a method of making a non-sag tungsten filament. The method entails irradiating the tungsten to form therein a very large number of inert gas atoms after a relatively massive sintered ingot has been formed and before the elongated tungsten filamentary member has been heated to a condition of incandescence.